Images are normally displayed by a CRT type display using an additive format such as in the well known Red, Green, Blue (RGB) format. In this format, separate colour information is provided for each of the Red, Green and Blue primary colours and when displayed together, the colours are mixed to form any desired colour to be displayed.
However, when it is desired to print out a colour representation of the image displayed by a colour printing device, a subtractive colour format is used. One such subtractive format is the Cyan, Magenta, Yellow and Black format (CMYK). Such a subtractive format is commonly used in colour printing device and in particular is used in Canon CLC 500 colour laser printer manufactured by Canon Inc.
A colour conversion apparatus is needed to convert the Red, Green, Blue format into the Cyan, Magenta, Yellow and Black format. However, the conversion of colour from one format to another presents a number of problems. One such problem occurs in the printing of black textual characters on a white background. Because the output of the black character may not be totally made up of only Black toner output, but also portions of Cyan, Magenta and Yellow toners, registration errors can occur on the borders of black to white transitions whereby the relevant toner does not mix properly and the individual colours are evident under close examination.
As the printing of black text on white backgrounds is a common operation with colour printing devices, it is desirable to substantially eliminate the occurrences of this unsightly output.
It is therefore an object of the present invention to substantially overcome, or ameliorate this problem.